The present invention relates in general to an RF heating process and associated apparatus. More particularly, the present invention relates to an RF heating process employed in the making or forming of molded plastic parts or pieces, particularly items made of plastic foams such as polyethelene or polyproplene foam.
The present technique for forming foam articles is illustrated in FIG. 1 herein in which there is schematically illustrated a mold 10 having a cavity for receiving the unprocessed foam block 12. Herein, for the sake of simplicity, the article being manufactured is simply of cubic form. However, it is understood that both the prior art technique and the technique of the present invention may be used in forming varied size and shape articles.
Associated with the mold 10 which may be of silicone rubber, are a pair of electrodes 14 and 16. The electrode 14 may be a movable electrode while the electrode 16 may be fixed. FIG. 1 schematically illustrates an RF source 18 coupling across the electrodes 12 and 14. FIG. 1 illustrates the foam block 12 having been inserted into the mold with the electrodes then moved to the position shown in FIG. 1. The RF field is then applied by source 18 to cause heating of the foam for the purpose of curing it and forming it into the desired shape. The foam is preferrably free-formed prior to insertion into the mold and has a shape that already substantially conforms to the shape of the mold. When the electrodes 14 and 16 are closed so as to ready the apparatus for heating, there may be a slight compression of the foam block 12. However, the prior art technique has never taught any substantial compression of the foam, particularly the art has not shown compression of the foam for the purpose of substantially reducing the heating and curing time thereof.
In the arrangement illustrated in FIG. 1 the typical heating time which represents some portion of the overall manufacturing cycle time, is typically in the order of about five minutes. It is over this five minute interval that RF energy is applied for heating. The mold is preferably constructed so as to have a high dielectric constant so that as much heat as possible is dissipated in the material that is being formed with less material being dissipated in the mold itself.
It is an object of the present invention to provide an improved RF heating process and associated apparatus in which the article can be formed over a heating period that is of substantially reduced duration. In accordance with the present invention it has been found that the heating period can be reduced from the afore mentioned typical five minute interval to on the order of 11/2 minutes. This is accomplished in accordance with the present invention by an RF heating process which combines RF heating with the use of a pair of electrodes, in combination with, substantial compression of the foam material with the foam material being maintained compressed during the heating cycle when the RF energy is applied. It has been found that by carrying out this compression while applying RF energy, that the heating time can be substantially reduced. After the heating cycle is over the previously compressed foam is permitted to expand as it cools and it then conforms to the desired over all mold shape.
In accordance with the apparatus of the present invention, in order to provide the aforementioned compression of the foam material, there may be provided a plug or the like. This plug can also be constructed of silicone rubber preferably loaded to have a high dielectric constant. This plug is temporarily used for compression of foam and is maintained in use during the heating cycle. The plug is then removed to enable expansion of the foam into its desired final shape.